In recent years, nonaqueous electrolyte secondary batteries have been increasingly demanded in industries such as backup power sources and storages for renewable energy from solar power generation and wind power generation in light of issues such as unreliable electricity supply. In these fields, the nonaqueous electrolyte secondary batteries are required to have high capacity and a long life through large-current cycles.
Increasing the density of an electrode is known as a technique for achieving a battery with high capacity. Compression by pressing, dense packing by controlling the size and shape of an active material, and the like are performed. However, in the case of increasing pressed density by pressing, there is a problem in that a long life through large-current cycles is unlikely to be achieved because diffusion paths of lithium ions are reduced.
In order to achieve a long life through large-current cycles, the adhesion between a mix layer and collector of an electrode and the adhesion of an active material in the mix layer are preferably strong. However, this requires the amount of a binder to be increased. Therefore, the capacity of a battery may possibly be sacrificed.
As described above, high capacity and a long life through large-current cycles are often in a trade-off relationship. Therefore, electrodes are prepared by controlling the composition or structure of an active material, a conductive agent, or an additive such as a binder such that battery characteristics suitable for applications are obtained. For example, Patent Literatures 1 to 4 propose that plate properties are controlled using a plurality of active material layers.
However, recent nonaqueous electrolyte secondary batteries are required to have various properties and therefore proposals described in Patent Literatures 1 to 4 are insufficient.